Method and apparatus for identifying co-channel interference

ABSTRACT

Methods and apparatuses identifying a co-channel interference signal in communications systems are disclosed. An exemplary method comprises generating an interference signal by subtracting a reconstructed desired signal from an at least partially demodulated composite signal, and generating synchronization statistics of interference signal using different scrambling codes. The interference signal is identified as the signal associated with the scrambling code that was used to generate an interference signal having a desired synchronization statistic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/102,958, filed Apr. 11, 2005, for “PHYSICAL LAYER HEADERSCRAMBLING IN SATELLITE BROADCAST SYSTEMS,” by Lin-Nan Lee, Feng-WenSun, Adam Von Ancken, Joseph Santoru, Ernest C. Chen, Shamik Maitra,Dennis Lai, and Guancai Zhou, which claims the benefit of the earlierfiling date under 35 U.S.C. §119(e) of U.S. Provisional Application Ser.No. 60/561,418 filed Apr. 12, 2004, entitled “CO-CHANNEL INTERFERENCEMITIGATION FOR DVB-S2,” both of which applications are herebyincorporated herein by reference.

This application is also related to the following applications, each ofwhich are hereby incorporated by reference herein:

U.S. patent application Ser. No. 11/103,307, filed Apr. 11, 2005, for“METHODS AND APPARATUSES FOR MINIMIZING CO-CHANNEL INTERFERENCE”, byLin-Nan Lee, Feng-Wen Sun, Adam Von Ancken, Joseph Santoru, Ernest C.Chen, Dennis Lai, and Guancai Zhou and Tung-Sheng Lin, which claimsbenefit of the earlier filing date under 35 U.S.C. §119(e) of U.S.Provisional Application Ser. No. 60/561,418, filed Apr. 12, 2004,entitled “CO-CHANNEL INTERFERENCE MITIGATION FOR DVB-S2”;

U.S. patent application Ser. No. 11/449,912, filed Jun. 9, 2006, for“METHOD AND APPARATUS FOR MINIMIZING CO-CHANNEL INTERFERENCE”, byLin-Nan Lee, Feng-Wen Sun and Adam Von Ancken, which is a continuationof U.S. patent application Ser. No. 11/009,346, filed Dec. 10, 2004, for“METHOD AND APPARATUS FOR MINIMIZING CO-CHANNEL INTERFERENCE”, byLin-Nan Lee, Feng-Wen Sun, Adam Von Ancken, issued Jan. 9, 2007 as U.S.Pat. No. 7,161,988, which claims benefit of the earlier filing dateunder 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No.60/561,418, filed Apr. 12, 2004, entitled “CO-CHANNEL INTERFERENCEMITIGATION FOR DVB-S2”;

U.S. patent application Ser. No. 11/009,333, filed Dec. 10, 2004, for“METHOD AND APPARATUS FOR MINIMIZING CO-CHANNEL INTERFERENCESCRAMBLING”, by Feng-Wen Sun and Iyer, which claims benefit of theearlier filing date under 35 U.S.C. §119(e) of U.S. ProvisionalApplication Ser. No. 60/583,410, filed Jun. 28, 2004, entitled“SCRAMBLING OF PHYSICAL LAYER HEADER AND PILOT SYMBOL IN DBV-S2 TOREDUCE CO-CHANNEL INTERFERENCE,” and Provisional Application Ser. No.60/585,654, filed Jul. 6, 2004, entitled “SCRAMBLING OF PHYSICAL LAYHEADER AND PILOT SYMBOL IN DVB-S2 TO REDUCE CO-CHANNEL INTERFERENCE”;and

U.S. patent application Ser. No. 11/102,983, filed Apr. 11, 2005, for“SHIFTED CHANNEL CHARACTERISTICS FOR MITIGATING CO-CHANNELINTERFERENCE”, by Joseph Santoru, Ernest C. Chen, Shamik Maitra, DennisLai, Guancai Zhou, and Tung-Sheng Lin, which claims benefit of U.S.Provisional Application Ser. No. 60/561,418, filed Apr. 12, 2004, andentitled “CO-CHANNEL INTERFERENCE MITIGATION FOR DVB-S2”; and

U.S. patent application Ser. No. 11/510,244, for “METHODS ANDAPPARATUSES FOR DETERMINING SCRAMBLING CODES FOR SIGNAL TRANSMISSION,”by Judith Wang, Guangcai Zhou, Joseph Santoru, Ernest C. Chen, ShamikMaitra, Dennis Lai, and Tang-Sheng Lin, filed Aug. 26, 2006, whichclaims benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent No.60/711,475 for “METHODS AND APPARATUSES FOR DETERMINING SCRAMBLING CODESFOR SIGNAL TRANSMISSION,” filed Aug. 26, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to communication systems, and moreparticularly to methods and apparatuses for minimizing signalinterference.

2. Description of the Related Art

FIGS. 1A and 1B illustrate a typical satellite based broadcast systemsof the related art.

FIG. 1A shows a communications system, specifically a televisionbroadcasting system 20, which transmits and receives audio, video, anddata signals via satellite. Although the present invention is describedin the context of a satellite-based television broadcasting system, thetechniques described herein are equally applicable to other methods ofprogram content delivery, such as terrestrial over-the-air systems,cable-based systems, and the Internet. Further, while the presentinvention will be described primarily with respect to television content(i.e. audio and video content), the present invention can be practicedwith a wide variety of program content material, including videocontent, audio content, audio and video related content (e.g.,television viewer channels), or data content.

Television broadcasting system 20 includes transmission station 26,uplink dish 30, at least one satellite 32, and receiver stations 34A-34C(collectively referred to as receiver stations 34). Transmission station26 includes a plurality of inputs 22 for receiving various signals, suchas analog television signals, digital television signals, video tapesignals, original programming signals and computer generated signalscontaining HTML content. Additionally, inputs 22 receive signals fromdigital video servers having hard discs or other digital storage media.Transmission station 26 also includes a plurality of timing inputs 24,which provide electronic schedule information about the timing andcontent of various television channels, such as that found in televisionschedules contained in newspapers and television guides. Transmissionstation 26 converts the data from timing inputs 24 into program guidedata. Program guide data may also be manually entered at the site oftransmission station 26. The program guide data consists of a pluralityof “objects”. The program guide data objects include data forconstructing an electronic program guide that is ultimately displayed ona user's television.

Transmission station 26 receives and processes the various input signalsreceived on inputs 22 and timing inputs 24, converts the receivedsignals into a standard form, combines the standard signals into asingle output data stream 28, and continuously sends output data stream28 to uplink dish 30. Output data stream 28 is a digital data streamthat is typically compressed using MPEG2 encoding, although othercompression schemes may be used.

The digital data in output data stream 28 are divided into a pluralityof packets, with each such packet marked with a service channelidentification (SCID) number. The SCIDs are later used by receiver 64(shown in FIG. 1B) to identify the packets that correspond to eachtelevision channel. Error correction data is also included in outputdata stream 28.

Output data stream 28 is a multiplexed signal that is modulated bytransmission station 26 using standard frequency and polarizationmodulation techniques. Output data stream 28 preferably includes 16frequency bands, with each frequency band being either left polarized orright polarized. Alternatively, vertical and horizontal polarizationsmay be used.

Uplink dish 30 continuously receives output data stream 28 fromtransmission station 26, amplifies the received signal and transmits thesignal 31 to at least one satellite 32. Although a single uplink dishand satellite are shown in FIG. 1, multiple dishes and satellites arepreferably used to provide additional bandwidth, and to help ensurecontinuous delivery of signals.

Satellites 32 revolve in geosynchronous orbit about the earth.Satellites 32 each include a plurality of transponders that receivesignals 31 transmitted by uplink dish 30, amplify the received signals31, frequency shift the received signals 31 to lower frequency bands,and then transmit the amplified, frequency shifted signals 33 back toreceiver stations 34.

Receiver stations 34 receive and process the signals 33 transmitted bysatellites 32. Receiver stations 34 are described in further detailbelow with respect to FIG. 1B.

FIG. 1B is a block diagram of one of receiver stations 34, whichreceives and decodes audio, video and data signals. Typically, receiverstation 34 is a “set top box,” also known as an Integrated ReceiverDecoder (IRD), which is usually resident in a home or multi-dwellingunit, for reception of satellite broadcasted television signals.Receiver dish 60 can be an Outdoor Unit (ODU), which is usually asmaller dish antenna mounted on a home or multi-dwelling unit. However,receiver dish 60 can also be a larger ground-mounted antenna dish ifdesired.

Receiver station 34 includes receiver dish 60, alternate content source62, receiver 64, monitor 66, recording device 68, remote control 86 andaccess card 88. Receiver 64 includes tuner 70/demodulator/Forward ErrorCorrection (FEC) decoder 71, digital-to-analog (D/A) converter 72, CPU74, clock 76, memory 78, logic circuit 80, interface 82, infrared (IR)receiver 84 and access card interface 90. Receiver dish 60 receivessignals 33 sent by satellite 32, amplifies the signals 33 and passes thesignals 33 on to tuner 70. Tuner 70 and demodulator/FEC decoder 71operate under control of CPU 74.

The CPU 74 operates under control of an operating system stored in thememory 78 or within an auxiliary memory within the CPU 74. The functionsperformed by CPU 74 are controlled by one or more control programs orapplications stored in memory 78. Operating system and applications arecomprised of instructions which, when read and executed by the CPU 74,cause the receiver 64 to perform the functions and steps necessary toimplement and/or use the present invention, typically, by accessing andmanipulating data stored in the memory 78. Instructions implementingsuch applications are tangibly embodied in a computer-readable medium,such as the memory 78 or the access card 88. The CPU 74 may alsocommunicate with other devices through interface 82 or the receiver dish60 to accept commands or instructions to be stored in the memory 78,thereby making a computer program product or article of manufactureaccording to the invention. As such, the terms “article of manufacture,”“program storage device” and “computer program product” as used hereinare intended to encompass any application accessible by the CPU 74 fromany computer readable device or media. Memory 78 and access card 88store a variety of parameters for receiver 64, such as a list ofchannels receiver 64 is authorized to process and generate displays for;the zip code and area code for the area in which receiver 64 is used;the model name or number of receiver 64; a serial number of receiver 64;a serial number of access card 88; the name, address and phone number ofthe owner of receiver 64; and the name of the manufacturer of receiver64.

Access card 88 is removable from receiver 64 (as shown in FIG. 1B). Wheninserted into receiver 64, access card 88 is coupled to access cardinterface 90, which communicates via interface 82 to a customer servicecenter (not pictured). Access card 88 receives access authorizationinformation from the customer service center based on a user'sparticular account information. In addition, access card 88 and thecustomer service center communicate regarding billing and ordering ofservices.

Clock 76 provides the current local time to CPU 74. Interface 82 ispreferably coupled to a telephone jack 83 at the site of receiverstation 34. Interface 82 allows receiver 64 to communicate withtransmission station 26 as shown in FIG. 1A via telephone jack 83.Interface 82 may also be used to transfer data to and from a network,such as the Internet.

The signals sent from receiver dish 60 to tuner 70 are a plurality ofmodulated Radio Frequency (RF) signals. The desired RF signal is thendownconverted to baseband by the tuner 70, which also generates in-phaseand quadrature (I and Q) signals. These two signals are then passed tothe demodulator/FEC Application Specific Integrated Circuit (ASIC) 71.The demodulator 71 ASIC then demodulates the I and Q signals, and theFEC decoder correctly identifies each transmitted symbol. The receivedsymbols for Quaternary Phase Shift Keying (QPSK) or 8PSK signals carrytwo or three data bits, respectively. The corrected symbols aretranslated into data bits, which in turn are assembled in to payloaddata bytes, and ultimately into data packets. The data packets may carry130 data bytes or 188 bytes (187 data bytes and 1 sync byte).

In addition to the digital satellite signals received by receiver dish60, other sources of television content are also preferably used. Forexample, alternate content source 62 provides additional televisioncontent to monitor 66. Alternate content source 62 is coupled to tuner70. Alternate content source 62 can be an antenna for receiving off theair National Television Standards Committee (NTSC) signals, a cable forreceiving Advanced Television Standards Committee (ATSC) signals, orother content source. Although only one alternate content source 62 isshown, multiple sources can be used.

Initially, as data enters receiver 64, CPU 74 looks for initializationdata which is referred to commonly in the industry as a boot object. Aboot object identifies the SCIDs where all other program guide objectscan be found. Boot objects are always transmitted with the same SCID, soCPU 74 knows that it must look for packets marked with that SCID. Theinformation from the boot object is used by CPU 74 to identify packetsof program guide data and route them to memory 78.

Remote control 86 emits Infrared (IR) signals 85 that are received byinfrared receiver 84 in receiver 64. Other types of data entry devicesmay alternatively be used, by way of example and not limitation, such asan ultra-high frequency (UHF) remote control, a keypad on receiver 64, aremote keyboard and a remote mouse. When a user requests the display ofa program guide by pressing the “guide” button on remote control 86, aguide request signal is received by IR receiver 84 and transmitted tologic circuit 80. Logic circuit 80 informs CPU 74 of the guide request.In response to the guide request, CPU 74 causes memory 78 to transfer aprogram guide digital image to D/A converter 72. D/A converter 72converts the program guide digital image into a standard analogtelevision signal, which is then transmitted to monitor 66. Monitor 66then displays the TV video and audio signals. Monitor 66 mayalternatively be a digital television, in which case no digital toanalog conversion in receiver 64 is necessary.

Users interact with the electronic program guide using remote control86. Examples of user interactions include selecting a particular channelor requesting additional guide information. When a user selects achannel using remote control 86, IR receiver 84 relays the user'sselection to logic circuit 80, which then passes the selection on tomemory 78 where it is accessed by CPU 74. CPU 74 performs an MPEG2decoding step on received audio, video, and other packets from FECdecoder 71 and outputs the audio and video signals for the selectedchannel to D/A converter 72. D/A converter 72 converts the digitalsignals to analog signals, and outputs the analog signals to monitor 66.

Such communications systems 20, here by example which is shown atelevision broadcast system 20, have embraced the demand for highquality transmissions made possible by digital technology. As thepackets and other data are transmitted from uplink dish 30 to receiver64, the symbols and bits in packets intended for other receiver stations34 are typically transmitted down from satellite 32 to receiver 64 onthe same frequency, because the transmit frequency is controlled by thelimitations of satellites 32, and the transmit frequencies that areavailable are controlled by government permission for transmission atspecific frequencies within the frequency spectrum.

Further, the data frames are coded in such a manner that they caninterfere with each other, and receiver 64 cannot tell which packets ofdata that receiver 64 is supposed to decode and present on monitor 66.Such interference is called “co-channel” interference, where one channelof data interferes with the reception and demodulation of anotherchannel of data. In practical applications, the co-channel interferencemay also stem from transmission of other system operators, a satellite32 operating in an adjacent orbital slot, or other spot transmissionbeams in a spot beam satellite broadcasting system 20.

As communications systems 20 transmits more data, i.e., more channels ofprogramming on a satellite broadcast system that are viewable on monitor66, the interference between data packets will increase, and, as such,the quality of the signal reception will be poorer.

To make optimal use of the available spectrum and to deliver a highnumber of different channels of programming, RF transmissions with thesame frequencies may be directed to different geographic areas. Howeverin areas bordering the different service areas, it is possible that areceiving station may detect a wanted transmission, but also otherco-frequency transmissions. The unwanted transmissions are interferenceand may severely degrade the overall performance of the wanted channelreceiver.

Traditionally, the negative effects of co-channel interference have beenminimized by redesigning the frequency assignments assigned to thevarious transponders or satellites 32. But this will not alleviate theproblem beyond a certain point. From the foregoing, it is apparent thatthere is a need to minimize interference beyond which, interferenceshould be minimized. The minimization of such interference is madeeasier if the interfering signal can be identified from the backgroundnoise. There is therefore a need for a method and apparatus foridentifying even weak interfering signal.

SUMMARY OF THE INVENTION

To minimize the limitations in the prior art, and to minimize otherlimitations that will become apparent upon reading and understanding thepresent specification, the present invention discloses methods andapparatuses for identifying a co-channel interfering signal.

In one embodiment, the method comprises the steps of (a) demodulatingthe composite signal to produce the desired data, (b) remodulating thedesired data to generate a reconstructed desired signal, (c) subtractingthe reconstructed desired signal from an at least partially demodulatedcomposite signal to generate the interference signal, (d) at leastpartially demodulating the interference signal using a first scramblingcode to produce a first demodulated interference signal, (e) computing astatistic of the demodulated interference signal, (f) repeating steps(d) through (e) to generate a plurality of statistics of the demodulatedsignal, one statistic for each of a plurality of scrambling codes and(g) identifying the interference signal according to a comparison of theplurality of statistics.

In one embodiment, the apparatus comprises a system for identifying aninterference signal from a received composite signal comprising adesired signal having desired data and an interference signal comprisinginterference data. The system comprises a demodulator for demodulatingthe composite signal to produce the desired data, a remodulator, coupledto the demodulator for remodulating the desired data to generate areconstructed desired signal, a subtractor, coupled to the remodulator,the subtractor for subtracting the reconstructed desired signal from anat least partially demodulated composite signal to generate theinterference signal, a timing recovery loop, coupled to the subtractor,the timing recovery loop for generating a plurality of at leastpartially demodulated interference signals from the interference signal,each of the plurality of at least partially demodulated interferencesignals generated with one of a plurality of associated scramblingcodes; and a signal analyzer, coupled to the second demodulator forcomputing a statistic for each of the plurality of at least partiallydemodulated interference signals and for identifying the interferencesignal according to a comparison of the plurality of statistics.

Still other aspects, features, and advantages of the present inventionare inherent in the systems and methods claimed and disclosed or will beapparent from the following detailed description and attached drawings.The detailed description and attached drawings merely illustrateparticular embodiments and implementations of the present invention,however, the present invention is also capable of other and differentembodiments, and its several details can be modified in variousrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature, and not as a restriction on the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIGS. 1A and 1B illustrate a typical satellite based broadcast systemsof the related art;

FIG. 2A is a diagram of a digital broadcast system capable of minimizingco-channel interference, according to an embodiment of the presentinvention;

FIG. 2B is a diagram of an exemplary transmitter employed in the digitaltransmission facility of the system of FIG. 2A;

FIG. 3 is a diagram of an exemplary demodulator in the system of FIG.2A;

FIGS. 4A and 4B are diagrams, respectively, of a frame structure used inthe system of FIG. 2A, and of logic for scrambling the frame headerswith different Unique Words (UWs) for respective frames transmitted overadjacent co-channels, in accordance with an embodiment of the presentinvention;

FIG. 5 is a diagram of a scrambler for isolating co-channel interferenceaccording to various embodiments of the present invention;

FIG. 6 is a diagram of an exemplary scrambling sequence generator usedin the scrambler of FIG. 5;

FIG. 7 is a diagram showing the periodic nature of the cross-correlationbetween co-channel frames, in accordance with an embodiment of thepresent invention;

FIG. 8 is a flowchart of a process for generating different physicallayer sequences, according to an embodiment of the present invention;

FIG. 9 is a flowchart of process for generating scrambled physicalheaders, according to an embodiment of the present invention;

FIG. 10 is a flowchart of process for transmitting scramblingparameters, according to an embodiment of the present invention;

FIG. 11 is a diagram showing various embodiments of the presentinvention for managing scrambling parameters;

FIG. 12 is a flowchart for descrambling received frames based onpre-designated sets of scrambling parameters, according to an embodimentof the present invention;

FIGS. 13A and 13B are flowcharts presenting illustrative processes thatcan be used to transmit information;

FIGS. 14 and 15 are diagrams depicting a representative technique foridentifying co-channel interference, and an embodiment of an apparatusthat can be used to perform the technique;

FIG. 16 is a diagram illustrating exemplary statistics for a partiallydemodulated interference signal; and

FIG. 17 is a diagram illustrating an exemplary computer system that canbe used to implement aspects of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An apparatus, method, and software for reducing co-channel interferencein a digital broadcast and interactive system are described. In thefollowing description, reference is made to the accompanying drawingswhich form a part hereof, and which show, by way of illustration,several embodiments of the present invention. It is understood thatother embodiments may be utilized and structural changes may be madewithout departing from the scope of the present invention.

Overview

In the present invention, the digital data transmitted from transmissionstation 26 via signal 31, satellites 32, and signal 33 contains threemain components: a header portion of a data frame, called the physicallayer header, or PL header, and payload data, and optionally, additionalinserted symbols, called pilot symbols, which are used by the receiver64 to mitigate the deleterious effects of degradation in the receiverstation 34, primarily phase noise. By using the PL header, thedemodulator/FEC-decoder 71 can quickly acquire the correct phase at thebeginning of every data frame. For many 8PSK and QPSK transmissionmodes, pilot symbols are also needed to track the phase noise moreaccurately. However, in certain instances, when the PL headers for adesired signal and an interfering co-frequency signal align in time, theinterference is so great that the demodulator/FEC-decoder 71 cannotdetermine with necessary accuracy the phase of the carrier frequencyassociated with the wanted signal. This means that as the demodulator 71tries to maintain a phase lock on the desired signal, the undesiredsignal presents the same header symbols or pilot symbols, and thedemodulator 71 can be confused by the presence of the undesired signal,and therefore unable to track the phase of the desired signal. Suchconfusion in the demodulator 71 is known in the art as having thedemodulator 71 being “pulled off” of the desired signal. If thedemodulator 71 is pulled by 45 degrees from the optimal constellationpoint for a QPSK transmission, the demodulator will not identify thesymbols correctly. This will introduce errors, and if not rectifiedquickly, the data errors will be identified as a loss of lock. This, inturn, will lead the microprocessor 74 to command the demodulator 71 toreacquire the signal, which leads to loss of data until the desiredsignal is reacquired. Such a loss of data would present incorrect dataon monitor 66, and possibly a service interruption on monitor 66 asviewed by a viewer. Rather than viewing a desired television channelwith motion and dialog on a given monitor 66, the co-channelinterference would cause the viewer to see the monitor fade to a darkscreen, or see a garbled picture, or hear garbled audio. It is apparentthat co-channel interference can create deleterious effects on atelevision broadcast system 20.

The present invention provides several factors that will mitigate theeffect of such co-channel interference.

A first approach is to provide a different Start-Of-Frame (SOF) sequenceand/or scrambling code to those channels that may be affected by suchco-channel interference. The demodulator 71 can then look for a specificSOF when asked to tune to one or the other of the data frames, and beable to tell the difference between them. Alternatively, or inconjunction, the codes used to scramble such interfering signals can besufficiently different that the cross-correlation between the two dataframes is reduced to the point where the demodulator 71 can lock ontothe desired transmission and disregard the deleterious effect of theinterfering channel. Further, different scrambling techniques can beused for PL Headers on different channels, and/or different scramblingtechniques or codes can be applied to the payload data, either inconjunction with scrambling of the PL Headers or separate from the PLHeaders, which will reduce or eliminate the pulling-off effect.

Another method to reduce co-channel interference effects is to sensewhen a demodulator 71 is being drawn away from tracking a specific phaseof a given signal. Such a drawing away, or “pulling off” of the phasetrack would indicate the presence of the interfering data frame, and thedemodulator 71 can then choose not to update the phase track from the PLheader or the pilot symbols. Another method of the present invention isto offset the transmission frequency of the modulated RF signal by asmall amount, e.g., 1 MHz, so the demodulator 71 can search for the SOFportion of the PL header in a different frequency space for a given dataframe. The number of offsets, and in which direction, e.g., either up ordown in terms of frequency, can be based on the number of independent RFtransmissions, or satellite 32 downlink beams, that will be presentsimultaneously and potentially causing the co-channel interference.Further, the data frames within a signal can also be offset in terms oftime, e.g., one data frame starts first, and the interfering data frameis delayed by a certain number of symbols, such that the SOF portion ofthe PL header will occur at different times for each of the data frames.This will protect the desired signal demodulator 71 from being pulledoff by the simultaneous presence of a PL header from an interferencesignal

Another method of the present invention is to use different shift keymodes within each of the data frames. Typically, a QPSK transmissionmode will be more resistant to co-channel interference effects than an8PSK transmission mode.

System Diagram

FIG. 2A is a diagram of a digital broadcast system 100 capable ofminimizing co-channel interference, according to an embodiment of thepresent invention. The digital communications system 100 includes adigital transmission facility 101 that generates signal waveforms forbroadcast across a communication channel 103 to one or more receivers105. According to one embodiment of the present invention, thecommunication system 100 is a satellite communication system thatsupports, for example, audio and video broadcast services as well asinteractive services. Such a communications system is shown in FIGS. 1Aand 1B, and described hereinabove. Interactive services include, forexample, electronic programming guides (EPGs), high-speed internetaccess, interactive advertising, telephony, and email services. Theseinteractive services can also encompass such television services as PayPer View, TV Commerce, Video On Demand, Near Video On Demand and AudioOn Demand services. In this environment, the receivers 105 are satellitereceivers. Satellite receivers are typically resident in “set topboxes,” also known as Integrated Receiver/Decoders (IRDs), which mayinclude digital video recorders (DVRs).

In broadcast applications, continuous mode receivers 105 are widelyused. Codes that perform well in low signal-to-noise (SNR) environmentsare at odds with these receivers 105 with respect to synchronization(e.g., carrier phase and carrier frequency). Physical layer headerand/or pilot symbols can be used for such synchronization. Accordingly,an important consideration with respect to system performance is that ofco-channel interference on physical layer header and/or pilot symbols.Because physical layer header and/or pilots are used for acquiringand/or tracking carrier phase and carrier frequency, such interferencecan degrade receiver performance.

Many digital broadcast systems 100 require use of additional trainingsymbols beyond that of the normal overhead bits in a frame structure fortheir synchronization processes. The increase in overhead isparticularly required when the Signal-to-Noise (SNR) is low; such anenvironment is typical when high performance FEC codes are used inconjunction with high order modulation. Traditionally, continuous modereceivers utilize a feedback control loop to acquire and track carrierfrequency and phase. Such approaches that are purely based on feedbackcontrol loops are prone to strong Radio Frequency (RF) phase noise andthermal noise, causing unacceptable cycle slip rates and an error flooron the overall receiver performance. Thus these approaches are burdenedby increased overhead in terms of training symbols for certainperformance target, in addition to limited acquisition range and longacquisition time. Further, these conventional synchronization techniquesare dependent on the particular modulation scheme, thereby hinderingflexibility in use of modulation schemes.

In system 100, the receivers 105 achieve carrier synchronization byexamining the preambles, headers, and/or unique scrambling codes orunique words (UW) that are embedded in broadcast data frame structures(shown in FIG. 4A), thereby reducing the use of additional overheadspecifically designated for training purposes. The receivers 105 aremore fully described below with respect to FIG. 3.

In this discrete communications system 100, the transmission facility101 produces a discrete set of possible messages representing mediacontent (e.g., audio, video, textual information, data, etc.); each ofthe possible messages has a corresponding signal waveform. These signalwaveforms are attenuated, or otherwise altered, by communicationschannel 103. To combat the noise in the broadcast channel 103, thetransmission facility 101 utilizes forward-error-correction codes, suchas Low Density Parity Check (LDPC) codes, or a concatenation ofdifferent FEC codes.

The LDPC or other FEC code or codes that are generated by thetransmission facility 101 facilitate high speed implementation withoutincurring any performance loss. These structured LDPC codes output fromthe transmission facility 101 avoid assignment of a small number ofcheck nodes to the bit nodes already vulnerable to channel errors byvirtue of the modulation scheme (e.g., 8PSK). Such LDPC codes have aparallelizable decoding process (unlike turbo codes), whichadvantageously involves simple operations such as addition, comparisonand table look-up. Moreover, carefully designed LDPC codes do notexhibit any sign of error floor, e.g., there is no decrease in errorseven though the signal-to-noise ratio increases. If an error floor wereto exist, it would be possible to use another code, such as aBose/Chaudhuri/Hocquenghem (BCH) code or other codes, to significantlysuppress such error floor.

According to one embodiment of the present invention, the transmissionfacility 101 generates, using a relatively simple encoding technique asexplained below in FIG. 2, LDPC codes based on parity check matrices(which facilitate efficient memory access during decoding) tocommunicate with the satellite receiver 105.

Transmitter Functions

FIG. 2B is a diagram of an exemplary transmitter employed in the digitaltransmission facility of the system 100 of FIG. 2A. A transmitter 200 intransmission facility 101 is equipped with an LDPC/BCH encoder 203 thataccepts input from an information source 201 and outputs coded stream ofhigher redundancy suitable for error correction processing at thereceiver 105. The information source 201 generates k signals from adiscrete alphabet, X. LDPC codes are specified with parity checkmatrices. Encoding LDPC codes requires, in general, specifying thegenerator matrices. BCH codes are included to reduce the error floor ofsystem 20, which improves error correction performance.

Encoder 203 generates signals from alphabet Y to a modulator 205, usinga simple encoding technique that makes use of only the parity checkmatrix by imposing structure onto the parity check matrix. Specifically,a restriction is placed on the parity check matrix by constrainingcertain portion of the matrix to be triangular for fast encoding anddecoding. Such a restriction results in negligible performance loss, andtherefore, constitutes an attractive trade-off.

Scrambler 209 scrambles the FEC encoded symbols in accordance with thepresent invention to minimize co-channel interference, as will be morefully described below.

Modulator 205 maps the scrambled messages from scrambler 209 to signalwaveforms that are transmitted to a transmit antenna 207, which emitsthese waveforms over the communication channel 103. The transmissionsfrom the transmit antenna 207 propagate to a demodulator, as discussedbelow. In the case of a satellite communication system, the transmittedsignals from the antenna 207 are relayed via a satellite.

Demodulator

FIG. 3 is a diagram of an exemplary demodulator/FEC decoder 71 in thesystem of FIG. 1. The demodulator/FEC decoder 71 comprises a demodulator301, a carrier synchronization module/descrambler 302, and a LDPC/BCHdecoder 307 and supports reception of signals from the transmitter 200via antenna 303. According to one embodiment of the present invention,the demodulator 301 provides filtering and symbol timing synchronizationof the LDPC encoded signals received from antenna 303, and carriersynchronization module 302 provides frequency and phase acquisition andtracking and descrambling of the signals output from the demodulator301. After demodulation, the signals are forwarded to a LDPC decoder307, which attempts to reconstruct the original source messages bygenerating messages, X′.

With respect to the receiving side, if both the desired and interferingcarriers use the same modulation and coding configuration (or mode),when the frame header (shown in FIG. 4A) are aligned exactly in timewhile their relative frequency offset are small, the interference cancause significant errors in phase estimation for the demodulator. As aresult, the demodulator can put out errors periodically. This conditionoccurs when frequency and symbol clock of the signals in question aresufficiently close, although they may be drifting with respect to eachother.

Frame Structure

FIG. 4A is a diagram of an exemplary frame structure used in the systemof the present invention. By way of example, an LDPC coded frame 400,which can support, for example, satellite broadcasting and interactiveservices, is shown. The frame 400 includes a Physical Layer Header(denoted “PL Header”) 401, which occupies one slot, as well as otherslots 403 for data or other payload. In addition, the frame 400,according to one embodiment of the present invention, utilizes a pilotblock 405 after every 16 slots to aid synchronization of carrier phaseand frequency. It is noted that the pilot blocks 405 are optional.Although shown after 16 slots 403, the pilot block (or pilot sequence)405, which can represent a scrambled block, can be inserted anywherealong the frame 400.

In an exemplary embodiment, the pilot insertion process inserts pilotblocks every 1440 symbols. Under this scenario, the pilot block includes36 pilot symbols. For instance, in the physical layer frame 400, thefirst pilot block is thus inserted at the end of 1440 payload symbolsafter the PL Header 401, the second pilot block is inserted after 2880payload symbols, and etc. If the pilot block position coincides with thebeginning of the next PL Header 401, then the pilot block 405 is notinserted.

The carrier synchronization module 302 (FIG. 3), according to anembodiment of the present invention, utilizes the PL Header 401 and/orpilot block 405 for carrier frequency and phase synchronization. The PLHeader 401 and/or pilot block 405 may be used for carriersynchronization, i.e., for assisting with the operation of frequencyacquisition and tracking, and phase tracking loop. As such, the PLHeader 401 and pilot block 405 are considered “training” or “pilot”symbols, and constitute, individually or collectively, a training block.

Each PL header 401 typically comprises a Start Of Frame (SOF) sectioncomprising 26 symbols, and a Physical Layer Signaling Code field (PLScode) field comprising 64 symbols. Typically, the SOF section isidentical for all PL headers 401 for all of the signals beingtransmitted without further scrambling.

For QPSK, 8PSK, and other modulations, the pilot sequence 405 is a36-symbol long segment (with each symbol being (1+j)/√{square root over(2)}); that is, 36 symbols (PSK). In the frame 400, the pilot sequence405 can be inserted after 1440 symbols of data. Under this scenario, thePL Header 401 can have 64 possible formats depending on the modulation,coding and pilot configuration.

When the PL headers 401 of the interfering carrier and the desiredcarrier (i.e., co-channels) are aligned in time, the coherentcontribution from the interfering PL Header 401 can introducesignificant phase error, causing unacceptable degradation inperformance. Likewise, if both co-channels use pilot symbols (with bothusing the same Gold code sequence for the pilot blocks 405), the pilotblocks 405 will be scrambled exactly the same way such that the coherentcontribution of the pilot block in the interfering carrier (orco-channel) is still problematic.

To mitigate the effect of co-channel interference, the frame 400 isscrambled, in pilot mode. In general, in this mode, the non-headerportion 407 is scrambled with a Gold code sequence unique to thetransmitter. This compares with a broadcast mode of the Digital VideoBroadcast S2 Standard (DVB-S2), for example, in which the entire frame400, including the pilot block 405, is scrambled using a common code;e.g., all the receivers 105 are supplied with the same Gold sequence.The scrambling process is further explained with respect to FIGS. 4B, 5,6, 8 and 9. As used herein, the scrambled pilot sequence is also denotedas a “pilot-segment” of the frame 400.

I and Q Swapping

Another method that can be used in accordance with the present inventionis to swap the in-phase (I) and quadrature phase (Q) portions of onesignal while leaving the co-channel phases intact. Such a phase swapwill destroy phase coherence in the co-channel data frames 400, whichminimizes or prevents interference between the two data frames 400 inthe co-channels.

Applying Different Scrambling Codes to the PL Header

As seen in FIG. 4B, to reduce the impact of co-channel interference,several different Unique Word (UW) patterns of the same length as the PLheader 401 can be utilized for the respective co-channels to scramblethe PL headers 401. For example, an eXclusive-OR (via an XOR logic 409)of the different UW patterns 411, 413 with the PL HEADER 401 can beperformed for the desired and interfering carriers (i.e., co-channels).Under this approach, power associated with the PL Header 401 of theinterfering carrier no longer adds coherently to the PL Header 401 ofthe desired carrier.

Although the frame 400 is described with respect to a structure thatsupports satellite broadcasting and interactive services (and compliantwith the DVB-S2 standard), it is recognized that the carriersynchronization techniques of the present invention can be applied toother frame structures.

Further, individual PL headers 401 can be scrambled prior to attachingthe PL header 401 to the frame 400, and individual PL headers 401 can bescrambled without other PL headers 401 being scrambled. The inventionenvisions selecting scrambling codes (or seeds to generate thescrambling codes), or, alternatively, selecting no scrambling code,based on the expected co-channel interference between two data frames400. The PL headers can be again scrambled as part of the data frame 400scrambling as shown in FIG. 5, or otherwise encrypted using anencryption schema.

The codes 411 and 413 that are used to scramble the PL header 401 can beGold codes as described herein, other seeded codes, or other codingschemes, without departing from the scope of the present invention. Suchcodes, or seeds for such codes, can be selected from a limited number ofcodes or seeds, and such codes or seeds can be sent to receiver 64 foruse in descrambling the data frames 400 to demodulate and descramble theframes 400. The limited number of codes or seeds can be selected basedon a number of factors, including the number of satellites 32, or thenumber of expected co-channel interferences in communication system 100.

Co-Channel Scrambling

FIG. 5 is a diagram of a sequence scrambler for isolating co-channelinterference, according to an embodiment of the present invention. Ascrambling code is a complex sequence that can be constructed from aGold code, according to one embodiment of the present invention. Thatis, a scrambler 209 generates a scrambling sequence Rn(i). Table 1defines how the scrambling sequence Rn(i) scrambles the frame using thescrambler 209, according to the scrambler sequence generator of FIG. 6.In particular, Table 1 shows the mapping of an input symbol to an outputsymbol based on the output of the scrambler 209.

TABLE 1 Rn(i) Input(i) Output(i) 0 I + jQ I + jQ 1 I + jQ −Q + jI 2 I +jQ −I − jQ 3 I + jQ Q − jI

Using different seeds for either of such two n-sequence generators cangenerate different Gold sequences. By using different seeds fordifferent services, the mutual interference can be reduced.

In a broadcast mode, the 90 symbol physical layer header 401 can remainconstant for a particular physical channel. The Gold sequence is resetat the beginning of each frame, and thus, the scrambled pilots areperiodical as well with a period equal to the frame length. Because theinformation carrying data in a frame varies and appears to be random,the co-channel interference is random and degrades the operatingsignal-to-noise ratio. Without using this scheme, due to the nature oftime-invariance of the original physical layer header 401 and the pilotblock 405, the carrier and phase estimation will be skewed for areceiver depending on these pilots and physical layer header for suchacquisition and tracking. This will degrade the performance beyond thoseof signal-to-noise ratio degradation associated with random data.

The scrambler 209 utilizes different scrambling sequences (n in FIG. 6)to further isolate the co-channel interference. One scrambling sequenceis provided for the physical layer header and one for the pilots.Different pilots are specified in terms of different seeds from the nvalue of the Gold sequences.

As such, the present invention contemplates separate scrambling ofseveral combinations of PL headers 401, pilot blocks 405, and payload403 for co-channel interference mitigation. Depending on the complexityof the system, the PL headers 401 and pilot blocks 405 (if present) fora given channel can be scrambled using a different code than theco-channel without scrambling the payload 403. In essence, allnon-payload 403 symbols that are present in one channel 400 arescrambled using one code, and all non-payload 403 symbols in anotherchannel 400 are scrambled using a different code.

Further, the PL headers 401 and pilot blocks 405 (if present) for twodifferent channels can be scrambled using different scrambling codes,and the payloads 403 for those channels can be scrambled using othercodes. For example, a first scrambling sequence can be applied to afirst PL header 401, and a second scrambling sequence can be applied toa second PL header 401. The first payload 403 has a third scramblingsequence applied (typically a Gold code), and the second payload has afourth scrambling sequence applied (also typically a Gold code).

It is also contemplated within the present invention that there can besystems that use mated pairs of codes for the PL header 401 and thepayload 403. So, a given scrambling code used on a PL header 401 isalways used with a scrambling code used to scramble the payload 403 forthat PL header 401. These code pairs can be applied to any signal 400,and can be re-assigned from one signal 400 to another signal 400 asdesired.

It is also contemplated within the scope of the present invention thateach payload 403 signal within system 20 receives a unique scramblingcode. Further, each PL header 401 can receive a unique scrambling code,which can be mated with scrambling codes for the payloads 403 ifdesired.

Although described as a single scrambling sequence for a given channel400, the present invention also contemplates that scrambling sequencescan be changed or rotated after a given number of frames have beentransmitted. The scrambling sequences for the PL header 401, the payload403, or both can be rotated on a random or periodic basis as desiredwithout departing from the scope of the present invention.

Gold Sequence Generator Diagram

FIG. 6 is a diagram of an exemplary scrambling sequence generator usedin the scrambler of FIG. 5. Although a Gold sequence generator is shownin FIG. 6, other sequence generators can be used within the presentinvention without departing from the scope of the present invention. Byusing different sequences for the co-channels, i.e., differentinitialization seeds for each of the co-channels, the interference canbe mitigated. In this example, a Gold sequence generator 700 employs thepreferred polynomials of 1+X⁷+X¹⁸ and 1+Y⁵+Y⁷+Y¹⁰+Y¹⁸. For example, tosustain n co-channels, in an exemplary embodiment of the presentinvention, the seeds can be programmed into an m-sequence generator 701.The polynomials are initialized based on the given seed for thatco-channel. The seeds are generated, according to one embodiment of thepresent invention, using a search algorithm that minimizes the worstcross-correlation between every pair of the co-channel pilot-segments.

Generating Different PL Sequences

FIG. 8 is a flowchart of a process for generating different physicallayer sequences, according to an embodiment of the present invention. Instep 801, different initialization seeds are assigned to the respectiveco-channels. Next, Gold sequences are generated based on the seeds, perstep 803. A scrambling sequence is then constructed, as in step 805,from the Gold sequence for each different service. In step 807, thephysical layer sequences are output by the scrambler 209.

The present invention can use different initialization seeds for each ofthe channels, and, thus, any pilot signals 405 in each signal willcontain different symbols, which greatly reduces cross-correlationbetween two interfering co-channels. Once the pilot symbols 405 aredistinguishable, the demodulator 71 can track one data frame 400 basedalmost entirely on the pilot symbols 405, which minimizes theinterference between the data frames 400.

FIG. 9 is a flowchart of process for generating scrambled physicalheaders, according to an embodiment of the present invention. Thetransmitter 200 (of FIG. 2A) receives input symbols associated with thephysical header or pilot sequence, as in step 901. In step 903, thetransmitter maps the input symbols according to a scrambling sequencegenerated by the scrambler 209. The output symbols are then generated,per step 905. Thereafter, the transmitter outputs a frame with ascrambled physical and/or scrambled pilot sequence (step 907).

FIG. 10 is a flowchart of process for transmitting scramblingparameters, according to an embodiment of the present invention. Asdiscussed above, for the pilot mode, different Gold sequences areemployed for different services to reduce co-channel interference. Inaddition, use of different UW patterns of the same length as the header401 can minimize coherent addition of the headers 401. Consequently, areceiver needs the appropriate UW to unscramble the PL Header 401, aswell as the appropriate Gold sequence to unscramble the payload data andthe pilot block.

In step 1001, the transmitter (e.g., transmitter 200) sends scramblingparameters for each of the supported carriers (co-channels) to receiver64. This is typically done by embedding the scrambling parameters intothe Advanced Program Guide (APG) portion of payload 403, which isavailable on at least one transponder from satellites 32. Typically, theAPG portion of payload 403 is available on every transponder fromsatellites 32, and receiver 64 can be directed to receive the APG on aspecific transponder on startup if such a direction to receiver 64 isnecessary. Further, the transmitter 200 can use other methods fortransmitting the scrambling codes, such as via telephone lines thatinteract with receiver 64 via interface 82. According to one embodimentof the present invention, the scrambling parameters include an index ofthe scrambling codes, and the scrambling sequence number for eachcarrier or channel. The default carrier supports a frame whose PL Header401 is not scrambled and the payload data 403 (and pilot block 405 ifany) are scrambled by a default Gold sequence, e.g., Sequence No. 0. Thereceiver 65, as in step 1003, initially tunes to this carrier to obtainthe scrambling parameters, and stores the scrambling parameter sets forall carriers to be received (per step 1005). When the receiver switchesto another carrier, as in step 1007, the particular scramblingparameters for the carrier are retrieved, per step 1009. In particular,the stored index is retrieved to find the correct UW as well as thestored Gold sequence number. In step 1011, the frames received over theparticular carrier are descrambled appropriately.

FIG. 11 is a diagram showing various embodiments of the presentinvention for managing scrambling parameters. In this example, asatellite system 20 includes a transmission station 26 that stores thescrambling parameters 1100 in external memory, i.e., a database 1102,for all carriers utilized in the system 20. The scrambling parameterscan be conveyed to receiver stations 34A-34C via satellites 32 using twoapproaches.

Under the first approach, the receiver 34 maintains all sets ofscrambling parameters that correspond to the carriers that is assignedto the receiver 34. In this manner, the transmission station 26 needonly indicate the particular entry associated with the proper set ofscrambling parameters for the receiver 34 to use for a particularcarrier. An update command only indicates the indices for these UW andGold sequence number in the database 1102 of the receiver 34.

The second approach employs a caching mechanism for pre-selected orpre-designated scrambling parameter entries, as explained in FIG. 12. Assuch, the receiver 34 includes a memory 78 to store the pre-designatedset of parameters.

FIG. 12 is a flowchart for descrambling received frames based onpre-designated sets of scrambling parameters, according to an embodimentof the present invention. With this approach, k sets of scramblingparameters corresponding to the carriers to be used by the receiver 34are pre-selected or pre-designated, as in step 1201. In other words,only k pre-selected UWs and k Gold sequence numbers are stored in atable. The value of k can be configured according to the size of thememory 78. As a result, the transmission station 26 need only transmit 2log₂ k bits for each carrier. Further, if a fixed association between UWand Gold sequence number is maintained, the number of transmitted bitscan be further reduced—one log₂ k bit number for each carrier. Thereceiver 34, thus, stores only k sets of scrambling parameters in thememory 78, per step 1203.

With this “cache” concept, the receiver 34 need not be instructed as toa particular set of scrambling parameter by the transmission station 26.At this point, if the receiver 34 determines that the transmissionstation 26 has indicated such instruction, per step 1205, the receiver34 retrieves the appropriate scrambling parameter from the memory 78 anddescrambles frames received over the specific carrier, as in step 1207.

Alternatively, the receiver 34 can, itself, determine a valid entry, asin step 1209, in the scrambling parameter table within the memory 78,assuming that k is sufficiently small as to not overburden theprocessing capability of the receiver 34. The receiver 34 can execute asearch procedure to step through all the possible k pre-selected sets ofUW and Gold sequence numbers stored in the memory 78, without receivingthese parameters via a default carrier, when the receiver first tunes toa particular carrier. Once the valid or correct set of UW and Goldsequence number is found for a particular carrier after the search, theinformation can be stored, per step 1211, in the memory 78 for thiscarrier. This information is then utilized to descramble the frame (step1213). Consequently, this valid set of scrambling parameters is used inthe future without further search when needed.

Under the above approach, great flexibility is afforded to how thescrambling parameters are conveyed to the receiver 34. The transmissionstation 26 can update the limited k UW and Gold sequence number setsthrough over-the-air programming. While there are k internal sets of UWand Gold sequence numbers stored in the memory 78 of the receiver 34,each of the sets can be replaced under remote command by thetransmission station 26 with a new UW and Gold sequence number. Forexample, in a cache update over-the-air, a full length of the UW, andthe Gold sequence number (e.g., 18-bits) along with the index istransmitted.

The processes of FIGS. 8-10 and 12 advantageously provide reducedco-channel interference, thereby enhancing receiver performance. Theseprocesses can be implemented as software and/or hardware, as explainedin FIG. 13.

Alternate Shift Key Modes

Another method of the present invention is to use different shift keymodes within each of the data frames 400. Typically, a QPSK transmissionmode will be more resistant to PL header 401 interference effects thanan 8PSK transmission mode. As such, some of the data frames 400 can betransmitted in a first PSK mode, and other frames 400 can be transmittedin a second PSK mode, which will reduce the number of bits/symbolswithin the data frames 400 that constructively interfere. Further,individual slots 403, pilot blocks 405, or PL headers 401 can betransmitted in different PSK or ASK modes to further reduce constructiveinterference, and, thus, reduce or eliminate co-channel interference.

Sensing Phase Track Pull-Off

Another method in accordance with the present invention to reduceco-channel interference effects is to sense when the demodulator 71 ortypically, carrier synchronization module 302 within the demodulator 71,is being abruptly or abnormally drawn away from tracking a specificphase of a given coded frame 400. Such a drawing away, or “pulling off”of the phase track would indicate the presence of the interfering dataframe, and the carrier synchronization module 302 can then choose not toupdate the phase track from the PL header 401 or the pilot symbols 405.Although the phase of a given signal or coded frame 400 can changeslowly, a reference phase track can be used by the carriersynchronization module 402 to maintain phase track of a given signal ifdesired.

As such, the present invention can use carrier synchronization module302 to determine the presence of an interfering coded frame 400, and caneither choose to update the carrier synchronization module 302 phasetracking information, or to ignore the phase tracking information, toallow carrier synchronization module 302 to track the already acquiredcarrier frequency for a given coded frame 400. The carriersynchronization module 302 can use statistical models or other methodsto determine how to track the phase of the desired coded frame 400rather than follow the phase tracking information caused by the presenceof the undesired and interfering coded frame 400.

Change in the SOF Sequence

The present invention also envisions that the interfering coded frames400 can have a different Start-Of-Frame (SOF) sequence and/or scramblingcode to those coded frames 400 that may be affected by such co-channelinterference. Typically, the SOF is the first twenty-six bits of theninety bit PL Header 401, but the SOF can be a larger or smaller amountof bits. Further, although changes in the SOF sequence are described,these techniques can be applied to any portion of the PL header 401 ifdesired. The demodulator 71 can then look for a different SOF in PLheader 401 when asked to tune to one or the other of the coded frames400, and be able to stay locked onto the desired signal and not bepulled off by co-channel interference.

Further, the different SOF sequences can be selected from a group of alimited number of SOF sequences, and this limited number of SOFsequences can be stored in receiver 64 such that receiver 64 can detector find a specific SOF sequence in a PL header 401 when required.

Transmission Frame Timing Offset

As shown in FIG. 7, it is possible to have two frames 601, 605 offset intime. The data frames 400 can be offset in terms of time as shown inFIG. 7, e.g., one data frame 400 starts first, and the interfering dataframe 400 is delayed by a certain portion of or whole number of symbols,such that the SOF portion of the PL header 401 will occur at differenttimes for each of the data frames, and not constructively interfere witheach other. This will allow the tuner 70 or demodulator 71 to know whichof the data frames 400 has been received based on the known time and/orfrequency offset for the data frames, or by processing the strongestsignal which is presumably the wanted signal, and then demodulate theproper data frame 400. The data frames 400 can be offset by any lengthlonger than one symbol interval.

Transmission Frequency Offset

Another method of the present invention is to offset the transmissionfrequency of data frames 601, 606 by a small amount, e.g., 1 MHz, so thedemodulator 71 can search for the SOF portion of the PL header 401 in adifferent frequency space for a given data frame 400. The number ofoffsets, and in which direction, e.g., either up or down in terms offrequency, can be based on the number of data frames 400, or satellite32 downlink beams, that will be present simultaneously and potentiallycausing the co-channel interference.

Information Transmission

FIGS. 13A-B are flowcharts presenting illustrative processes that can beused to transmit information using the foregoing principles.

FIG. 13A is a flowchart that presents illustrative steps in which theheaders of a first and a second signal are scrambled before transmissionover different channels. Box 1300 represents scrambling a first headerof the first signal using a first scrambling code. Box 1302 representsscrambling a second header of the second signal using a secondscrambling code. Box 1304 represents transmitting the first signal andthe second signal with the scrambled first header and the scrambledsecond header over different channels of the communication system.

FIG. 13B is a flowchart that presents illustrative steps in which theheaders and payload of the signals are scrambled using scrambling andgold codes, respectively. Box 1306 represents scrambling a first headerof the first signal using a first scrambling code. Box 1308 representsscrambling a first payload of the first signal using a first Gold code.Box 1310 represents scrambling a second header of the second signalusing a second scrambling code. Box 1312 represents scrambling a secondpayload of the second signal using a second Gold code. Box 1314represents transmitting the first signal and the second signal with thescrambled first header and the scrambled second header over differentchannels of the communication system.

Identification of Co-Channel Interference

Co-channel interference (CCI) can be introduced into a satellitebroadcasting network in several ways, including by geographicallyadjacent spot beam transmissions and cross-talk in different components,such as multiswitches, and cross-polarization. Described herein is amethod and system that can detect and possibly identify CCI signals if adominant CCI exists with an I/N of as low as −4 dB, where 1 representsthe interference power and N represents power from noise, lineardistortions, non-linear distortions and other impairments. If the CCIsignal is an advanced modulation signal and is coded using LDPC/BCHforward error correction, the technique can identify the interferingsignal by processing the statistics of frame synchronization based onthe unique scrambling code of the transmitted signal.

The system and method disclosed herein utilizes techniques devised todecode the layered modulation (LM) signal. In the LM technique(described, for example, in U.S. Pat. No. 7,209,524, which is herebyincorporated by reference herein) two signals are transmittedsimultaneously with identical or overlapping spectra. The two signalsmay use the same or different modulation and forward-error-correctionschemes, but the two signals have different powers. Processing works byfirst decoding the higher power signal. If error-free decoding of thesignal is successful, the data is re-encoded and the signal isremodulated. Additional impairments, such as carrier frequency offset,linear or nonlinear distortions can also be included in thereconstructed signal. The reconstructed waveform is then subtracted fromthe composite signal, leaving the lower-power, or interference signal,noise, uncompensated distortions and demodulation errors.

FIGS. 14 and 15 are diagrams depicting a representative technique foridentifying co-channel interference, and an embodiment of an apparatusthat can be used to perform the technique. In FIG. 15, the blocks withsolid borders (1504, 1506, 1508, 1510, 1512, 1514, 1516 and 1518) areassociated with the desired signal, the dash-dotted blocks (1520, 1524,1526, 1528 and 1530) are those that are associated with the extractionof the interference signal, and the bolded blocks (1532, 1534, 1536 and1538) are associated with the interference signal.

Referring to FIG. 14, the composite signal 1502 is demodulated, as shownin block 1404. The composite signal 1502 includes a desired signalhaving desired data and interfering signal. The demodulation of thecomposite signal includes the process of both timing recovery andcarrier recovery, hence demodulator 1503 comprises a timing recoverymodule 1504 and a carrier recovery module 1511 illustrated in FIG. 15.

The timing recovery module 1504 obtains the composite signal 1502 froman antenna 60, which may include a low noise block converter (LNB) forshifting the frequency of the received energy to lower frequencies. Thetiming recovery module 1504 includes an optional coarse frequencyestimator module 1506, a low pass filter module 1508, and a timingrecovery loop module 1510.

The coarse frequency estimator module 1506 reduces the uncertainty ofthe estimate of the carrier frequency of the signal. A delay andmultiply (DM) algorithm may be used for this module. The DM algorithm isbased on the principle that the carrier frequency can be estimated bythe phase difference between two adjacent time samples.

The low pass filter module 1508 low pass filters the signal from thecourse frequency estimator module to remove noise. The resulting signalis then be applied to an automatic gain control (AGC) module (not shown)so that the input to the timing recovery loop module 1510 is ofrelatively constant amplitude.

The signal is then applied to a timing recovery loop module 1510 toproduce an at least partially demodulated composite signal 1513. Thetiming recovery loop module 1510 obtains symbol synchronization. Toaccomplish this, the timing recovery loop module 1510 determines thesampling frequency, and the sampling phase. Determining and locking thesampling frequency requires estimating the symbol period so that samplescan be taken at the correct rate. Although the sampling frequency shouldbe known (e.g., the system's symbol rate is typically known), oscillatordrift will introduce deviations from the stated symbol rate. Determiningand locking the sampling phase involves determining the correct timewithin a symbol period to take a sample. Due to bandwidth and otherlimitations, real-world symbol pulse shapes typically have a peak nearthe center of the symbol period. Sampling the symbol at this peakresults in the best signal-to-noise-ratio and will ideally eliminateintersymbol interference from adjacent symbols.

Next, the at least partially demodulated signal 1513 is applied to thecarrier recovery module 1511 to complete the demodulation process. Thedemodulator 1511 includes a frame synchronizer 1512, a fine frequencyestimator 1514 and a carrier recovery loop (CRL) 1516.

The frame synchronizer 1512 finds the start of the frame (SOF) forincoming data frames based on the headers (90 symbols) of the physicallayer frame (PLFrame). The information required to determine SOF dependson the characteristics of the received signal. The SOF may be determinedfrom the pilot signal, the scrambling code, or other information. Theinformation required to determine the SOF (signal modulation type,whether the pilot is on or off, coding rates, etc.) is typically knownapriori.

As all symbols become available when the input signal is read andprocessed, the probability of frame detection is typically 99% at aCNR=1 dB, and the false detection probability is typically less than 1%.For a CNR as low as −4 dB, detection probability is about 36%. Thesestatistics can be used in the identification of the scrambling ID eventhough the signal being identified is deeply buried in background noise.

The output of the frame synchronizer 1512 is provided to a finefrequency estimation module 1514. This module further reduces thefrequency uncertainty to assure that the carrier recovery that followswill be relatively error free, especially for 8PSK modes with pilots.The output of this module is provided to the carrier recovery loop (CRL)module 1516.

The CRL module 1516 removes the residual sinusoidal carrier signal inthe composite signal 1502. Demodulating the received signal is mostlyaccomplished using a local oscillator and a mixer in the tuner. Ideally,the oscillator used to modulate the signal and the oscillator used inthe CRL module 1516 to demodulate the received signal are synchronizedin frequency and phase. However, in practice, the frequency of eitherthe modulating oscillator or the local oscillator may change or driftwith time. Therefore, instead of demodulation bringing the signal tobaseband, the signal will be near baseband with some frequency offset,causing the received signal constellation to rotate. The CRL module 1516removes this frequency offset using a closed loop system, thus allowingthe signal to be processed at baseband without any rotation.

If the composite signal 1502 was encoded, the output of the CRL module1516 is decoded by decoder 1518, thus producing the desired data.

Referring again to FIG. 14, the desired data is remodulated to generatea reconstructed desired signal 1527, as shown in block 1406. This can beaccomplished by remodulator 1522 shown in FIG. 15. If the compositesignal 1502 was coded, the desired data is re-encoded by encoder 1520before being provided to the remodulator 1522. The remodulator 1522remodulates the data to generate the reconstructed desired signal 1527.This reconstructed desired signal is later subtracted from the at leastpartially demodulated signal 1513. The reconstructed desired signal maybe optionally pulse shaped by the modulation and pulse shaping module1524 and compensated in both phase and frequency by the frequency andphase compensation module 1526 using information from the fine frequencyestimation module 1514 and the CRL module 1516.

Referring again to FIG. 14, the reconstructed desired signal issubtracted from the at least partially demodulated composite signal togenerate an interference signal 1531, as shown in block 1408. This canbe accomplished by the signal cancellation module 1530 depicted in FIG.15. In one embodiment, the signal cancellation module 1530 comprises asubtractor (1531) for subtracting the reconstructed desired signal 1527from the received signal 1513, and may also comprise further modulesthat compensate for transmission channel non-linearities anddistortions.

The processing required to perform the operations depicted in blocks1511 and 1522 can take an significant amount of time. Hence, thereceived signal 1513 may be delayed by delay module 1528 by the sameamount of time to assure that the subtraction of the reconstructeddesired signal 1527 from the received signal 1513 provides theinterference signal 1531 as desired.

To identify the signal, a plurality (N) of scrambling codes are used toat last partially demodulate the interference signal 1531, and thestatistics of a forced SOF detection resulting from the application ofeach scrambling code is examined. When the incorrect scrambling code isused, the resulting SOF will be randomly distributed in time, but whenthe correct scrambling code is used, the SOF will be systematicallydistributed in time.

The interference signal is at least partially demodulated, using a firstone of N scrambling codes to generate an at least partially demodulatedinterference signal, as shown in block 1410. The partial demodulation ofthe interference signal can be performed by blocks 1532-1538 of FIG. 15.In this embodiment, the interference signal is supplied to a coarsefrequency estimator 1532, which operates analogously to the coarsefrequency estimator 1506 of the timing recovery module 1504 in that itreduces the uncertainty of the estimate of the signal carrier frequency(in this case, the interference signal carrier). Next, the signal issupplied to a low pass filter 1534 and a timing recovery loop 1536, anda frame synchronizer 1538, which perform operations analogous to thoseof the low pass filter 1508, timing recovery loop 1510, and framesynchronizer 1512, respectively.

A statistic of the at least partially demodulated interference signal isgenerated as shown in block 1412. This can be accomplished, for exampleby the signal analyzer 1540 shown in FIG. 15.

Steps 1410-1412 are repeated with another one of the N scrambling codesuntil all candidate scrambling codes are attempted, as shown in blocks1412 and 1414. This results in a plurality of statistics for each of thecandidate scrambling codes. Finally, as shown in block 1416, theinterference signal is identified by comparing the statistics generatedfor each of the scrambling codes.

In one embodiment, the statistic of the at least partially demodulatedinterference signal is a synchronization statistic. For example, the atleast partially demodulated interference signal may comprise a pluralityof frames, each with a detected SOF time. In this case, the statisticgenerated for each scrambling code may be referenced to the start SOFtime from all subsequent SOFs of the partially demodulated signal.

FIG. 16 is a diagram illustrating exemplary statistics for the at leastpartially demodulated interference signal. The uppermost plot is ahistogram illustrating 12 time “bins” for the SOF time, and indicatesthe number of frames for which the SOF occurred within that time bin.For example, the top histogram indicates that more frames had a SOF inthe time interval corresponding to bin 1 than in bin 2. The greatestnumber of frames had a SOF time within bin 7 and the fewest within bin11. However, although the top plot includes a bin having the greatestnumber of frame (bin 7) and one with the fewest number of bins (bin 11),none of the bins has a substantially greater number of frames than theother bins. This is also the case with the middle histogram (generatedwith a second scrambling code). In contrast, the bottom histogram(generated for scrambling code M) shows that two of the bins (6 and 7)have significantly more frames than the rest (1-5 and 8-12). This is anindication that scrambling code M belongs to the interference signal.Using a table, database, or other relationship stored in the receiverthat maps the scrambling code to the signal, the interference signal canbe thus identified as the signal associated with scrambling code M.

In the above-described embodiment, histograms of the SOF timing werecompared to identify the interference signal. However, the presentinvention may be practiced using other statistics. For example, insteadof generating histograms of the SOF time, the system may simply generatea variance (or standard deviation) of the SOF time for each scramblingcode, and identify the interfering signal as the signal associated withthe scrambling code resulting in the lowest variance for the SOF time.

FIG. 17 illustrates an exemplary computer system 1700 that could be usedto implement the present invention. The computer 1702 comprises aprocessor 1704 and a memory, such as random access memory (RAM) 1706.The computer 1702 is operatively coupled to a display 1722, whichpresents images such as windows to the user on a graphical userinterface 1718B. The computer 1702 may be coupled to other devices, suchas a keyboard 1714, a mouse device 1716, a printer, etc. Of course,those skilled in the art will recognize that any combination of theabove components, or any number of different components, peripherals,and other devices, may be used with the computer 1702.

Generally, the computer 1702 operates under control of an operatingsystem 1708 stored in the memory 1706, and interfaces with the user toaccept inputs and commands and to present results through a graphicaluser interface (GUI) module 1718A. Although the GUI module 1718A isdepicted as a separate module, the instructions performing the GUIfunctions can be resident or distributed in the operating system 1708,the computer program 1710, or implemented with special purpose memoryand processors. The computer 1702 also implements a compiler 1712 whichallows an application program 1710 written in a programming languagesuch as COBOL, C++, FORTRAN, or other language to be translated intoprocessor 1704 readable code. After completion, the application 1710accesses and manipulates data stored in the memory 1706 of the computer1702 using the relationships and logic that was generated using thecompiler 1712. The computer 1702 also optionally comprises an externalcommunication device such as a modem, satellite link, Ethernet card, orother device for communicating with other computers.

In one embodiment, instructions implementing the operating system 1708,the computer program 1710, and the compiler 1712 are tangibly embodiedin a computer-readable medium, e.g., data storage device 1720, whichcould include one or more fixed or removable data storage devices, suchas a zip drive, floppy disc drive 1724, hard drive, CD-ROM drive, tapedrive, etc. Further, the operating system 1708 and the computer program1710 are comprised of instructions which, when read and executed by thecomputer 1702, causes the computer 1702 to perform the steps necessaryto implement and/or use the present invention. Computer program 1710and/or operating instructions may also be tangibly embodied in memory1706 and/or data communications devices 130, thereby making a computerprogram product or article of manufacture according to the invention. Assuch, the terms “article of manufacture,” “program storage device” and“computer program product” as used herein are intended to encompass acomputer program accessible from any computer readable device or media.

CONCLUSION

In summary, the present invention comprises methods and apparatuses forminimizing co-channel interference in communications systems. It isintended that the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto and theequivalents thereof. The above specification, examples and data providea complete description of the manufacture and use of the composition ofthe invention. Since many embodiments of the invention can be madewithout departing from the spirit and scope of the invention, theinvention resides in the claims hereinafter appended and the equivalentsthereof.

1. In a system receiving a composite signal comprising a desired signalhaving desired data and an interference signal having interfering data,a method of identifying the interference signal, comprising the stepsof: (a) demodulating the composite signal to produce the desired data;(b) remodulating the desired data to generate a reconstructed desiredsignal; (c) subtracting the reconstructed desired signal from an atleast partially demodulated composite signal to generate theinterference signal; (d) at least partially demodulating theinterference signal using a first scrambling code to produce a firstdemodulated interference signal; (e) computing a statistic of thedemodulated interference signal; (f) repeating steps (d) through (e) togenerate a plurality of statistics of the demodulated signal, onestatistic for each of a plurality of scrambling codes; and (g)identifying the interference signal according to a comparison of theplurality of statistics.
 2. The method of claim 1, wherein the statisticis a synchronization statistic.
 3. The method of claim 2, wherein: thestep of at least partially demodulating the interference signal usingthe first scrambling code to produce the first demodulated signalcomprises the step of recovering the timing of the interference signal;and the interfering data comprises a plurality of frames and statisticis a frame synchronization statistic.
 4. The method of claim 3, whereinthe step of computing the statistic of the demodulated interferencesignal comprises the step of generating a histogram of the framesynchronization characteristic of the demodulated interference signal.5. The method of claim 4, wherein the step of identifying theinterference signal according to a comparison of the plurality ofstatistics comprises the step of: identifying the interference signal asthe signal associated with a histogram having the lowest variance of theplurality of histograms.
 6. The method of claim 1, wherein: the step ofdemodulating the composite signal to produce the desired data comprisesthe step of demodulating and decoding the composite signal to producethe desired data; and the step of remodulating the desired data togenerate the reconstructed desired signal comprises the step of recodingand remodulating the desired data to generate the interference signal.7. A system for identifying a an interference signal from a receivedcomposite signal comprising a desired signal having desired data and aninterference signal comprising interference data, comprising: means fordemodulating the received composite signal to produce the desired data;means for remodulating the desired data to generate a reconstructeddesired signal; means for subtracting the reconstructed desired signalfrom an at least partially demodulated composite signal to generate theinterference signal; means for generating a plurality of at leastpartially demodulated interference signals from the interference signal,each of the plurality of at least partially demodulated interferencesignals generated with one of a plurality of associated scramblingcodes; means for computing a statistic for each of the plurality of atleast partially demodulated signals; means for identifying theinterference signal according to a comparison of the plurality ofstatistics.
 8. The system of claim 7, wherein: the means for at leastpartially demodulating the interference signal using the firstscrambling code to produce the first demodulated signal comprises meansfor recovering the timing of the interference signal; and the statisticis a synchronization statistic.
 9. The system of claim 8, wherein theinterfering data comprises a plurality of frames and statistic is aframe synchronization statistic.
 10. The system of claim 9, wherein themeans for computing the statistic of the demodulated interference signalcomprises a means for generating a histogram of the framesynchronization characteristic of the demodulated interference signal.11. The system of claim 10, wherein the means for identifying theinterference signal according to a comparison of the plurality ofstatistics comprises means for identifying the interference signal asthe signal associated with a histogram having the lowest variance of theplurality of histograms.
 12. The system of claim 7, wherein: the meansfor demodulating the composite signal to produce the desired datacomprises the means for demodulating and decoding the composite signalto produce the desired data; and the means for remodulating the desireddata to generate the reconstructed desired signal comprises means forrecoding and remodulating the desired data to generate the interferencesignal.
 13. A system for identifying an interference signal from areceived composite signal comprising a desired signal having desireddata and an interference signal comprising interference data,comprising: a demodulator for demodulating the composite signal toproduce the desired data; a remodulator, coupled to the demodulator forremodulating the desired data to generate a reconstructed desiredsignal; a subtractor, coupled to the remodulator demodulator, thesubtractor for subtracting the reconstructed desired signal from an atleast partially demodulated composite signal to generate theinterference signal; a timing recovery loop, coupled to the subtractor,the timing recovery loop for generating a plurality of at leastpartially demodulated interference signals from the interference signal,each of the plurality of at least partially demodulated interferencesignals generated with one of a plurality of associated scramblingcodes; and a signal analyzer, coupled to the second demodulator forcomputing a statistic for each of the plurality of at least partiallydemodulated interference signals and for identifying the interferencesignal according to a comparison of the plurality of statistics.
 14. Thesystem of claim 13, wherein the statistic is a synchronizationstatistic.
 15. The system of claim 14, wherein the interfering datacomprises a plurality of frames and statistic is a frame synchronizationstatistic.
 16. The system of claim 15, wherein the signal analyzercomprises generates a histogram of the frame synchronizationcharacteristic of each of the plurality of demodulated interferencesignals.
 17. The system of claim 16, signal analyzer identifies theinterference signal according to the comparison of the plurality ofstatistics by identifying the interference signal as the at leastpartially demodulated interference signal associated with a histogramhaving the lowest variance of the plurality of histograms.
 18. Thesystem of claim 13, further comprising: a decoder coupled between thedemodulator and the remodulator; and a recoder, coupled between thedemodulator and the subtractor.